Generally, in an image forming apparatus, such as a laser printer and the like, a developing cartridge, on which a developing roller is rotatably supported, is detachably mounted (for example, see JP-A-2003-295614).
In such a developing cartridge, an input gear for transmitting a driving force to the developing roller is provided on one side wall of a casing for containing the developing roller. The input gear engages with a developing roller driving gear of the developing roller. When the developing cartridge is mounted on a main body of the image forming apparatus, the input gear is coupled with a coupling member that is provided on the main body of the image forming apparatus, so that it transmits a driving force from the coupling member to the developing roller driving gear.
Further, in the developing cartridge, on the other side wall of the casing, an electrode member for applying a developing bias to the developing roller is provided. The electrode member is integrally formed with a bearing member that supports a developing roller shaft of the developing roller. When the developing cartridge is mounted on the main body of the image forming apparatus, the electrode member comes into contact with an electrode plate that is provided on the main body of the image forming apparatus, such that it applies a developing bias supplied from the electrode plate to the developing roller shaft.